1. Field of the Invention
The present invention relates to electrical and electronic circuits and systems. More specifically, the present invention relates to systems and methods for reducing crest factor in electrical and electronic circuits and systems.
2. Description of the Related Art
In Multi-Carrier Power Amplifier (MCPA) communication transmission applications, multiple carriers are typically combined in the baseband, intermediate frequency (IF) or radio frequency (RF) frequency range and the resulting signal is transmitted using a single power amplifier. An aspect for MCPA transmission is to transmit a signal at a very high efficiency while maintaining a low Adjacent Channel Power Ratio (ACPR) to meet spectral mask requirements. ACPR is defined as the ratio of power in a bandwidth away from the main signal (the distortion product) to the power in a bandwidth within the main signal. The bandwidths and locations are functions of the standards being employed.
To achieve high efficiency power amplifier (PA) transmission, it is desirable to use semi-non-linear PAs, such as Class A/B PAs. A challenge for MCPA signal transmission is due to the fact that the combined signal has a high crest factor (ratio of peak power to average power), where the peak power is significantly higher than the average power. A small portion of the combined signal can have very high peaks and when transmitted at high PA efficiency, these high-level signals reach into the saturated region of the PA's transfer function and the output of the PA has high intermodulation distortion (IMD). The high IMD level raises the ACPR levels.
To maintain low ACPR without any linearization techniques, the transmit signal level must be decreased sufficiently so that the peak amplitudes are not in the saturated zone of the PA, but this reduces the amplifier efficiency. For example, a four carrier W-CDMA (wideband code division multiple access) signal can have a crest factor exceeding 13 dB. If the crest factor is reduced by about 6 dB, the average power can be increased by 6 dB thus increasing the power efficiency by a factor of 4.
One approach to this problem is to limit the amplitude of either the baseband signal or the RF signal output of each channel using a look-ahead approach. However, it is difficult to generate signals with low crest factor and low ACPR inasmuch as limiting the amplitude increases out of band emissions (e.g. sidelobes) and thereby raises the ACPR level. Similarly, efforts to reduce the ACPR levels generally increase crest factor.
Another approach involves the use of unused CDMA codes to reduce the crest factor in the output signals. However, this approach requires knowledge of what is being transmitted so that the unused codes can be identified. This adds to the complexity, storage requirements and cost of the system.
Hence, a need remains in the art for an improved system or method for reducing the crest factor in communications systems while maintaining a low ACPR therefor.